The layers of a multi-layer foil have different tasks, for example if the foil is used as packaging material. The layer facing the product to be packaged must be chemically neutral to this product. At least one intermediate layer serves to provide the desired strength and the desired elasticity, especially when the foil is used for a packaging container. The outside layer must be able to carry print and should have a surface that is visually and physically attractive. In the case of a diffusible product it may also be necessary to reinitialize one of the intermediate layers as a diffusion-inhibiting barrier. For example, if one of the intermediate layers is of metal, then typically an adhesive layer on both sides of the metal is required to assure a reliable bond with the other layers. At least one of the outer layers is of a weldable or sealable plastic, so that the package to be produced can be sealed shut.
A tubular body intended for packaging tubes typically has a longitudinal seam, at which the overlapping edges of the multilayer foil are sealed together.
The inside surface adjacent one edge is typically sealed to the outside surface adjacent the other edge. Such a seam does form a tight closure, but it has the disadvantage that the inner layer, facing the product, is interrupted along the seam because of the multi-layer structure of the foil. This presents the risk that the packaged product may come into contact, in the vicinity of the seam, not only with the chemically neutral layer but with the other layers as well.